1. Field of Invention
This invention relates to the use of wind energy to drive a wind turbine for the production of electrical power, and specifically to a control system which automatically modulates the pitch angle of the wind turbine blades in order to regulate either electrical output power, shaft torque or speed in order to minimize the effects of wind gusts and turbulence, and to reduce stress on the blades and other mechanical components.
2. Description of the Prior Art
Attempts to harness the forces of nature for man's benefit is recorded in the earliest pages of history. One of the first practical applications was the use of wind energy to drive windmills in order to produce power. A concern that the world's available energy resources will eventually be depleted has resulted in renewed interest in the generation of power from naturally occurring phenomena, and has given rise to the development of various schemes to generate this power economically, efficiently and dependably. As a consequence, the windmill has received considerable attention as a partial solution to supplying the world's increasing energy demands.
The basic problem with windmill or wind turbine generated power is not its overall availability, but in harnessing this power in an efficient manner and supplying it in the proper form useful to electrical utilities or to isolated stations. In many locations winds are, at best, unpredictable as to direction and velocity, and the availability of useful output power at any given time is uncertain. The amount of power available varies with wind speed, and gusts cause transient changes in output power. While the windmill power output may be used directly to drive mechanical devices, its most useful form is electrical, in which form the power may be transmitted to new or existing power grids for use by industry and homes. To produce useful electrical power, the rotary energy of the windmill is used to drive a dynamo, which produces a.c. or d.c. electrical power as desired. In some applications d.c. output power is used to charge large storage batteries, the output from the storage batteries being used to provide electrical power when needed. The use of storage batteries generally necessitates conversion from d.c. to a.c. via static inverters or other means. If a.c. power is produced rather than d.c. via a windmill driven synchronous generator, both the frequency and phase of the a.c. power must generally be regulated, as well as the power output, before the a.c. power can be transmitted to commercial users, or fed into existing power grids.
It has been found that the control necessary to produce electrical power from a synchronous generator, driven by a wind turbine, can be provided by varying the pitch angle of the wind turbine blades, in a manner analogous to the blade pitch control for an aircraft propeller. U.S. Pat. No. 2,363,850 describes a wind turbine driven a.c. generator with a speed governor controlled mechanism for varying the angle of the wind turbine blades between fully feathered and fully powered positions. Means are described to regulate electrical output frequency, phase and power, and to disconnect the electrical generator at wind velocities which are too high or too low to produce the desired power. U.S. Pat. No. 2,547,636 provides an automatic speed control for a wind turbine to control the charging rate of a storage battery, the speed control consisting of mechanical devices responsive to wind velocity for changing the blade pitch angle. U.S. Pat. No. 2,583,369 is a similar control for mechanically adjusting blade pitch angle to maintain a relatively constant electrical generator speed, and hence a relatively constant a.c. output frequency.
U.S. Pat. No. 2,795,285 is directed to a control for varying the rate of change of load, speed or voltage of a wind driven motor by varying the pitch of the wind turbine blades in a closed loop manner. U.S. Pat. No. 2,832,895 is another device for adjusting the blade pitch of a wind turbine in response to a predetermined charge on a battery, or in response to sudden gusts of wind.
The basic problem with the prior art devices is that they do not act rapidly enough, or with sufficient accuracy, to limit stresses in blades and other mechanical components to acceptable levels. They are unduly affected by wind gusts and turbulence, and cannot maintain satisfactory power control over a wide range of wind conditions to allow connection to a conventional power grid or power distribution system. At high wind velocities even mild turbulence creates significant fluctuations in power, and may cause the generator to be removed from the power grid.
The present invention overcomes the limitations of the prior art, and provides a very responsive and rapidly acting pitch control for the blades of a wind turbine. The control maintains a.c. electrical frequency, phase, speed, torque and power within desired tolerances, and also schedules the blade pitch angle during start up and shutdown to prevent undesired loads on the mechanical components. The control is adaptive in that the blade angle controls are responsive to wind velocity magnitude and to changes in wind velocity to maintain satisfactory power, torque and speed control. The control system is preferably electronic and is therefore fast acting, and may be implemented inexpensively with digital computers or microprocessing equipment.
It is therefore an object of the present invention to provide an improved pitch angle control for a wind turbine which modulates the wind turbine blade angle in response to a large number of operating conditions.
Another object of the present invention is an electronic pitch angle control for wind turbines which schedules blade angle to minimize blade stress and shaft torque variations during start up and shutdown transients.
A further object of the present invention is an electronic pitch angle control for wind turbines which regulates speed, torque and power output of a turbine driven synchronous generator in a closed loop manner.
Another object of the present invention is a closed loop blade angle control for a wind turbine in which proportional, integral and derivative control signals are produced, and in which the loop gains are continuously varied as a function of wind velocity.
A further object of the present invention is an electronic control for maintaining the a.c. output from a wind turbine driven synchronous generator at a predetermined power, frequency and phase and which automatically regulates the connection of the a.c. power into a grid.
Another object of the present invention is a power generating system including a wind driven turbine which compensates the blade pitch angle control for rapid changes in the wind.
A further object of this invention is a closed loop control for a wind turbine driven generator in which the closed loop contains an integrator which automatically tracks the blade angle of the wind turbine even when the control is inactive.
Another object of this invention is a wind turbine driven generator system in which wind speed is synthesized from system operating parameters.
A further object of this invention is a wind turbine driven generator in which turbine blade angle is controlled as a function of either generator speed or generator power depending on the connection of the generator to a power transmission grid.